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Traction Control Systems Using electronics to make your car corner better! Many of the world’s car manufacturers are now adopting traction control systems for their vehicles. These systems, often fitted in conjunction with all-wheel drive, reduce the likelihood of a car leaving the road during cornering. By JULIAN EDGAR Many car manufacturers now have traction control systems and these come under a variety of names. Lexus use the acronym “VSC” for “Vehicle Stability Control”. Mitsubishi call it either “Active Yaw Control” or “Active Stability Control”, depending on which technical strategy is followed. Delphi (GM’s electronic arm) tag their system “Traxxar” for some incomprehensible reason, while Nissan uses Understeer Fig.1: understeer occurs when the front of the car slides first. An understeering car will tend to head straight on, rather than following the corner. 18  Silicon Chip the ghastly acronym “ATTESA ET-S” for their 4-wheel drive system which incorporates stability control. Finally, Mercedes Benz call such systems “ESP”, for “Electronic Stability Program”. Such is the sophistication of the system, it could stand for “Extra Sensory Perception” – and that’s as good a reason as any for sticking with the term “ESP” throughout this article. Oversteer Fig.2: oversteer occurs when the rear of the car slides first. An oversteering car will spin if no correction is made. Whew; that’s got the nomenclature out of the way! Good drivers, bad drivers A vehicle transmits all its cornering and acceleration forces through the contact areas of its tyres. Each of these contact “patches” is only about the area of a large shoe print and four of these must control a vehicle with a mass of perhaps 1.5 tonnes and travelling at speeds of 30m/s or more. Viewed in this light, it can be seen that hard braking, cornering and acceleration can be very much a balancing act – exceed the levels of grip provided by the tyres and regaining control could require a very skilled driver indeed. However, most of us aren’t skilled drivers, especially in an emergency situation where a combination of hard cornering and braking may be needed. This type of swerve, brake, recover situation often results in a complete loss of control, unless the driver is skilled at such manoeuvres. But what if an electronic system was constantly measuring and evaluating individual wheel speeds, steering input angle, vehicle yaw and vehicle acceleration? Such a system could react far faster than a human driver and, using algorithms developed through extensive testing, take the appropriate action to ensure vehicle stability. In short, it would eliminate those heart-stopping moments when the back of the car attempts to overtake the front – a boon for those who drive in icy conditions! It would also prevent loss of control if the road condition changes suddenly or if the driver makes an error, such as entering a corner too quickly. But do such systems work? Early in its ESP development, Mercedes Benz placed 80 of its vehicle owners in the Mercedes driving simulator in Berlin. At 100km/h, an icy situation was suddenly simulated on four road bends, the vehicle’s grip on the road decreasing by more than 70% within a few metres. Without any form of ESP, 78% of the drivers left the road. By contrast, when the ESP system was activated, all drivers safely negotiated the bends. Data collected by the General Motors Safety Center indicates that 29% of severe accidents in the USA are caused by loss of vehicle control. This means that ESP systems can play an important role in vehicle safety – both by negating the effects of driver behaviour and by allowing the driver to retain control in changing road conditions. Cornering behaviour Routine driving behaviour occurs well within the limits of tyre adhesion. This means that the cornering forces developed between the road and the tyres remain proportional to the tyre slip angles. It also means that, at a given speed, the yaw rate of the vehicle remains approximately proportional to the steering angle. However, if the vehicle speed or steering angle continues to increase, a point is reached where the cornering forces no longer increase. When this occurs, small changes in lateral forces can produce large changes in the slip angles of the front or rear tyres. Conversely, large changes in slip angles can result in little or no change in lateral forces. When the limits of adhesion are reached, a cornering vehicle behaves in two distinct ways. If the front tyres are the first to lose grip, the car is said to understeer. The behaviour of an understeering car is shown in Fig.1. The car leaves the road on the outside of the corner, because the front wheels are “under” steering; ie, not steering enough! Conversely, if the rear tyres lose grip first, the car oversteers. Fig.2 shows the path that an oversteering car takes. As can be seen, if no correction is undertaken, oversteer can result in a spin. It’s important to realise that the amount of lateral grip that a tyre can develop depends on both the cornering and acceleration loads placed on it (among other things). A powerful Fig.3: speed sensors are integrated into the hub of the car. Here the cable going to the sensor can be seen just to the left of the drive shaft. rear-wheel drive car may be prone to “power oversteer”, where lateral traction is lost because the rear tyres’ grip is overcome by the magnitude of the torque being applied. Under a combination of heavy braking and strong cornering, a loss of lateral grip will occur at much lower cornering accel­erations than if a steady speed was being maintained. These factors influence the ESP control strategy, which is most effective in active 4-wheel drive cars. any control corrections, it must know how the vehicle is currently behaving. It does this by using a number of sensors, which are distributed around the car. All cars fitted with ESP have an anti-lock braking system (ABS) fitted. This means that individual wheelspeed sensors are already present. It also makes it relatively easy to implement a system that controls the vehicle by separately braking individual wheels. In most vehicles, the speed sensors Signal inputs typically use a toothed wheel rotating Before an ESP system can perform past an inductive sensor. Fig.3 shows a Lexus speed sensor, as seen in its normal (installed) state. The cable going to the sensor can be seen just to the left of the driveshaft. In addition to speed sensing, ESP systems also require a means of detecting the steering angle, vehicle yaw rate and vehicle acceleration. The steering angle sensor detects the amount and direction of steering lock being applied. Lexus vehicles use an optical sensor to perform this function (see Fig.4). This particular device Fig.4: the Lexus steering angle sensor uses uses three photo interan optical design. Three sensors are used rupters, which work in in conjunction with a slotted disc. conjunction with a slotFebruary 1999  19 Coriolis Force Straightline Movement Side-to-Side Movement Fig.6: the Lexus GS300 yaw sensor. It is normally located beneath the centre console in the cabin. Detection Portion ω ω=0 ω Vibration Portion Coriolis Force Output Voltage The Lexus yaw rate sensor uses a piezoelectric vibration type rate gyro. The resonator is shaped like a tuning fork, with a vibrating portion and a Yaw Rate detecting portion mount­ Right Turn Left Turn ed at 90° to each other Fig.5: the Lexus yaw rate sensor uses a and located on each arm piezoelectric vibration type rate gyro. The of the fork – see Fig.5. To resonator is shaped like a tuning fork, with detect the yaw rate, an AC a vibrating portion and a detecting portion voltage is applied to the mounted at 90° to each other and located on vibrating portion, exciteach arm of the fork. To detect the yaw rate, ing it. During yaw motion, an AC voltage is applied to the vibrating the detecting portion of portion, exciting it. The detecting portion of the assembly is distorted the assembly is then distorted by a certain amount and direction by the Earth’s Coriolis by the Earth’s Coriolis force acting on the arms of the fork. force, which acts on the arms of the fork. The result is an output voltage from the sensor, ted disc. Two of the sensors detect which is proportional to the direction steering angle and direction, while and magnitude of the yaw rate. Fig.6 is the third is used to determine the a photograph of one of these sensors. neutral position of the steering wheel. As indicated earlier, the magnitude Self-checking mechanisms are built of acceleration (braking, acceleration into the sensor. or cornering) also influences the ESP The vehicle yaw rate is a critical control strategy that is selected. Veinput for ESP systems. The yaw rate hicles use an accelerometer to detect is the speed at which the vehicle is this characteristic. The Lexus accelturning around a vertical axis passing erometer is located in close proximity through the centre of the car. Yaw rate to the yaw sensor and consists of two sensors are usually positioned in the weighted semiconductor elements. middle of the car – directly behind These are mounted at 90° to one the gearshift lever in the case of the another, with each at 45° to the lonLexus models. However, the Delphi gitudinal axis of the car – see Fig.7. Traxxarä system locates this sensor The outputs from the two sensors are under the rear parcel shelf. fed to the ESP control unit, which 20  Silicon Chip calculates the horizontal acceleration in all directions. Depending on how the ESP system is integrated with other electronic systems in the car, additional sensors may be fitted to detect brake fluid pressure and throttle opening. In most cars, these sensors are already present and so they can be included in an ESP system for very little additional cost. Signal outputs The outputs of most ESP systems are used to actuate individual wheel brakes and reduce drivetrain torque to selected wheels. In no system is the steering angle automatically changed, so the wheel isn’t suddenly wrenched from your grip as the computer takes over! In 4-wheel drive cars, an ESP system changes the front/rear torque distribution, while one Mitsubishi model can even change the side-toside torque distribution! Many ESP systems use braking as their primary control mechanism. The Lexus GS300, for example, integrates the hydraulic aspects of the ESP, ABS and conventional braking systems into one package. Instead of having a separate hydraulic master cylinder, vacuum booster and ABS hydraulic control unit, these systems are all incorporated into one firewall-mounted assembly. An impressive array of hardware is built into this compact unit, as follows: (1) a pump and pump motor; Fig.7: the Lexus accelerometer uses two sensing elements mounted at 90° to each other, with the assembly at 45° to the longitudinal axis of the car. (2) a nitrogen-charged pressure accumulator; (3) three pressure switches; (4) a relief valve; (5) the brake fluid reservoir; (6) the master cylinder; (7) the brake booster, which applies accumulator pressure; (8) four switching solenoid valves, to direct fluid pressure to any or all of the wheels; and (9) four pressure control solenoid valves that regulate the hydraulic pressure applied to each wheel’s brake. A photograph of this marvel is shown in Fig.8. Note the small lifting hooks positioned on the assembly (we can only conclude that it’s installed using a small block and tackle)! Other vehicles in the Lexus range retain a more traditional approach but this integrated hydraulic unit clearly shows the way of the future. The engine torque is reduced by reducing the throttle opening. The Lexus models use electronically-controlled throttle bodies, so this is easily achieved. Other systems retard camshaft timing (when variable cam timing system is used), reduce the ignition advance or even bypass individual fuel injectors. Fig.9 shows a block diagram of the complete stability control system used in the Lexus GS300. Mitsubishi uses a Torque Transfer Differential in their Automatic Yaw Control system. This differential is able to regulate the amount of torque being transferred to each wheel on the one axle. Currently, only the rear axle can be controlled in this manner. The system works by using an electrically-controlled hydraulic unit which engages wet multi-plate clutches by varying amounts, to give the active torque split. Fig.10 shows the system, which is being used in 4-wheel drive performance cars and is said to be especially effective in sharp corners. Nissan’s ATTESA ET-S 4-wheel drive system has a similar wet multi-plate clutch system. It is used to distribute torque to the front wheels as required, to give maximum stability. Other outputs of an ESP system include self-diagnostic codes, a dash Fig.8 (below): the Lexus GS300 hydraulic assembly. It integrates the ABS hydraulic control unit, the brake booster and the control valves for the stability control system. February 1999  21 Fig.9: the Lexus GS300 stability control system. Inputs include wheel speeds, steering angle, deceleration and yaw rate. As indicated on the diagram, the same system is used for anti-lock brakes, traction control and vehicle stability control purposes. light (or gauge) to warn the driver when the system has activated, and another warning light to indicate that the system is inoperative. Control strategies Designing input sensors and output actuators for an ESP system is relatively straightforward but that doesn’t apply when it comes to writing the software. Developing ESP control algorithms that work effectively in all situations is apparently quite difficult. In fact, some systems have quite different software, depending on the market that the car is aimed at. Delphi, for example, use a different approach in the rear-wheel drive Chevrolet Corvette sports car to that used on several front-wheel drive Cadillac models. As with suspension 22  Silicon Chip tuning, what is best for one market sector is not necessarily best for another. That also implies another thing: when ESP systems become common, look out for “hot” programs that will be available on the aftermarket! When a vehicle is understeering, braking of the inside rear wheel substantially reduces the amount of understeer that occurs. This can be easily understood if you again look at Fig.1. The vehicle is attempting to negotiate a righthand bend but the front of the car is sliding wide. If the righthand rear wheel was slowed while the other wheels continued to turn at their normal rate, the car would attempt to pivot around this wheel to the right. This would allow the car to successfully negotiate the bend in the road, instead of under- steering off the road to the left. In the rear-wheel drive Lexus cars, both rear wheels are braked and the engine torque output is reduced – see Fig.11. Toyota presumably adopted this approach because the car is designed to initially understeer if the cornering speed is too great. Simply slowing the car thus provides the required reduced understeer. Research from Delphi has shown that braking the inside front wheel can also significantly correct understeer but this applies only at small slip angles. When a vehicle is oversteering, the most powerful corrective braking mechanism that can be employed is to brake the outside front wheel to near lock-up. In Fig.2, this would be the front lefthand wheel. If this wheel is braked but the others continue at normal speed, the car would attempt to pivot around to the left, thereby reducing the amount of oversteer. The Lexus system does just this but it’s not always quite that simple. At times, the Lexus also brakes the rear wheels during oversteer. This is likely to occur (in conjunction with a reduction in engine torque) when too much throttle is being applied. While the yaw change that occurs with the slowing of a single wheel is the major corrective mechanism, another factor also has a significant affect. Earlier, it was stated that the grip of a tyre depends on both the cornering and the longitudinal loads placed on it. When an ESP system is activated, the car is at the limits of adhesion and then one wheel is suddenly braked! The braked tyre will thus slide sideways more easily than it did before the braking loads were imposed. Let’s now take another look at the oversteering vehicle in Fig.12. When the front lefthand wheel is braked, its lateral grip is also reduced. This means that the car will have less front-end grip and so the front of the car will start to move to the left – ie, in the same direction that the back is heading! So this effect also acts to decrease oversteer. In an active 4-wheel drive car, the control strategy is based on reducing the amount of torque that’s transferred to the end of the car that’s sliding. For example, the Nissan Skyline GT-R is a rear-wheel drive car for most of the time. However, if power oversteer occurs during cornering, torque is transferred to the front wheels, thereby reducing the torque load on the rear tyres and also pulling the car in the steered direction. Some forms of the Nissan system do not use a yaw sensor, the torque split control being based only on the inputs received from accelerometers, wheel-speed sensors and the throttle position. With 2-wheel drive cars, a typical control algorithm consists of the following steps: (1) Calculate the desired values of vehicle yaw rate and slip angle, using the steering angle and vehicle speed; (2) Using the difference between the desired and measured yaw rates and between the desired and estimated slip angles, determine the desired change in yaw that should be applied to the vehicle; (3) Select the wheel(s) to which the brakes should be applied and determine the desired magnitude of braking pressure or brake slip. Fig.10: Mitsubishi’s Active Yaw Control allows the amount of torque being channelled through each rear wheel to be varied by means of a Torque Transfer Differential. Understeering Control Moment Oversteering Control Moment Braking Force Braking Force Fig.11: the Lexus system brakes both rear wheels to control understeer. Other systems brake just the inside rear wheel, creating a correcting yaw moment. Closed loop control can be used during braking so that maximum retardation of the chosen wheel occurs. This prevents the need for an estimation of the surface coefficient of friction. The major parts suppliers to vehi- Fig.12: oversteer in the Lexus is controlled by braking the outside front wheel (car shown here making a right turn). cle manufacturers have stated quite clearly that adding an ESP system to a car already equipped with ABS can be done quite cheaply. That makes it very likely that stability control technology will find its way into a wide range of cars in the near future. SC February 1999  23